It is known to provide a side door of a motor vehicle with a moveable side window often referred to as a ‘drop-down window’ or ‘drop-glass’. Increasingly the amount of tumblehome used for a motor vehicle is being increased for design and/or aerodynamic purposes. The use of a large tumblehome creates packaging problems for a drop-down window if a side window having a conventional curvature is used.
Automotive glazing on the side of a vehicle body consists of a number of elements that span the length of the side of the body. The glazing is usually designed using CAD software with the aim of harmonizing the reflections and enabling dropping of glazing elements to open the windows.
The final shape usually consists of a substantially horizontal arc of a suitable curvature along the beltline of the motor vehicle and a substantially vertical arc of suitable curvature near the B-pillar. These arcs are then swept into a 3D surface. If both arcs were of the same radius and had the same origin, the resulting surface would be a ball but in practice the radii are significantly different so that the resulting surface is usually barrel shaped. Therefore, the windows forming the side portions of the glasshouse each has an outer surface that is curved in two directions that is to say the surface is as though cut from a barrel or prolate spheroid.
The vertical arc is said to have a vertical barrel radius of curvature and is the curvature of a part of a motor vehicle shaped as though cut from a barrel when the part is bisected by a transverse vertical plane through the motor vehicle such as the vertical transverse plane V-V shown on FIG. 1a. 
FIG. 3a shows as a chain dotted line the path W1 of a conventional curved side window having a conventional relatively large vertical barrel radius of curvature. When such a side window (not shown) is in a raised or closed position it abuts against part of the structure of the motor vehicle such as a cantrail 29. It will be appreciated that the seals required between the conventional side window and the cantrail 29 have been omitted from FIG. 3a. The outline of a lower part 13 of a side door is shown in outline.
The conventional large vertical barrel radius of curvature window W1 has a chord length C2 above the lower part 13 of the door when in the raised position and an additional chord length C2″ (not shown for the window up position) that remains within the lower part 13 of the door to support the window W1. When in the fully lowered position the chord C2 is moved to the position C2′ and the chord C2″ is as shown.
The width of the lower part 13 of the side door will have a direct effect on the total vehicle width and passenger compartment width. The path W1 of the conventional side window crosses an outer boundary of the lower part 13 of the side door so as to lie a distance “x1” outside the outer boundary of the lower part 13. Therefore it would not be possible to lower the conventional side window along the path W1 unless the outer boundary of the lower part 13 is moved outwardly by a distance greater than “x1”. It is undesirable to move the outer boundary of the lower part 13 outwards because this will increase the overall width of the motor vehicle. This is particularly disadvantageous in the case of a compact motor vehicle such as a city car where the overall dimensions of the motor vehicle are critical.
FIG. 3a also shows as a chain dotted line the path W2 of an alternative side window having a smaller vertical barrel radius of curvature of the type required for a motor vehicle with a large tumblehome and having a vertical barrel radius of curvature chosen so as to prevent the path W2 from crossing the outer boundary of the lower part 13. As before, when the alternative side window (not shown) is in a raised or closed position it abuts against the cantrail 29.
The small vertical barrel radius of curvature window W2 has a chord length C3 above the lower part 13 of the door when in the raised position and an additional chord length C3″ (not shown for the window up position) that remains within the lower part 13 of the door to support the window. When in the fully lowered position the chord C3 is moved to the position C3′ and the chord C3″ is as shown.
In this case the smaller vertical barrel radius of curvature results in the path W2 of the alternative side window crossing an inner boundary of the lower part 13 of the side door so that a lower edge of the alternative side window would lie a distance “x2” inside the inner boundary of the lower part 13. Therefore it is not possible to lower the alternative side window along the path W2 unless the inner boundary of the lower part 13 is moved inwardly by a distance greater than “x2”. However, moving the inner boundary inwards is undesirable because it will affect the overall width of a passenger compartment of the motor vehicle. This is a particular problem if the motor vehicle is a compact motor vehicle such as a city car which is already likely to have a relatively narrow passenger compartment.
FIGS. 4a and 4b show, in a diagrammatic manner, how the problem of using a large tumblehome presents itself when a motor vehicle has side doors 110 that are thin and very contoured. In the case of compact motor vehicles such as a city car it is desirable to keep the thickness of the side doors 110 as thin as possible in order to maximise internal space while minimising external width of the motor vehicle. It is also common practice on modern motor vehicles to provide a significant concave contour to an outer surface 114 of a lower part 113 of the side door 110 for styling or aerodynamic purposes and to use such a door 110 in combination with a large tumblehome.
It can be seen that with a conventional single drop-down window 121 that has a large vertical barrel radius of curvature to match the tumblehome, the path of the window 121 indicated by the dotted line R-R will cause the window 121 to foul a door lock mechanism 150 and is not contained within the package envelope for the lower part 113 of the side door 110 when the window is lowered.
In FIGS. 5a and 5b a first solution to this problem is shown in which the contoured concave outer surface shown in FIGS. 4a and 4b is replaced by a convex outer surface 114b, where the lock mechanism 150 is moved outwards accordingly, and all other details remain the same. Although with such an arrangement the window 121 will not foul the door lock mechanism 150 and is contained within the package envelope for the lower part 113 of the side door 110 when the window is lowered it is unacceptable for many vehicles because the thickness of the side door is excessive thereby compromising vehicle interior space and the overall width of the motor vehicle. In addition the original contour has been replaced by a bulging convex contour which may not match the styling of the motor vehicle and may not provide the aerodynamic flow required for the motor vehicle.
In FIG. 6 a second solution to the problem set out in FIGS. 4a and 4b is shown. In this case the solution is to reduce the tumblehome so that instead of the drop-down window 121 following the path R-R it follows the path R′-R′ thereby enabling the window 121 to be packaged within the lower part 113 of the side door 110. The problem with this solution is that it restricts the choice of tumblehome available to the designer when shaping the character, style and proportion of the vehicle or the engineer when looking to improve the aerodynamic efficiency of the vehicle.